Capacitance sensors usually include a housing which includes two chambers, the first for receiving the gas or vapor whose pressure is to be measured and the second for receiving a gas at a reference pressure. The reference pressure is typically either determined from a reference source or ambient gas reflecting ambient conditions. The two chambers are separated by a flexible diaphragm positioned therebetween. The diaphragm is fixed around its periphery so that it seals one chamber from the other, and has an active portion that flexes as a function of the difference between the measured pressure and the reference pressure. In a capacitance manometer, the diaphragm is spaced from two electrodes, one typically a ring electrode and the other a center electrode, both axially aligned with the center axis of the active portion of the diaphragm. The electrodes are positioned in one of the chambers, usually the chamber containing the reference gas. When a differential pressure is applied to the diaphragm, the diaphragm flexes relative to the two electrodes. By electrically connecting the diaphragm so that it also functions as an electrode, the differential pressure can be measured as a function of the difference in capacitance between the diaphragm and the center electrode and the capacitance between the diaphragm and the ring electrode. With the reference pressure known, the measured pressure can easily be determined from the capacitance measurements.
The pressure measurement range of the capacitance manometer is partly determined by the spacing between the diaphragm and the ring and center electrodes. In the zero position, the surfaces of the diaphragm must be flat and as parallel as possible to the ring and center electrodes. Thus, the lower the values in differential pressure that are to be measured, the closer the diaphragm needs to be secured in the sensor housing relative to the ring and center electrodes to provide a maximum signal dynamic range for the pressure range in question. This results in the requirement of parallelism to be even greater. As a result great care is taken to lap the surfaces of the diaphragm prior to securing the diaphragm within the housing so that they are as planar as possible. Further, care must be taken to secure the diaphragm within the housing so as to ensure the proper position and parallelism of the diaphragm relative to the ring and center electrodes. The diaphragm is usually formed as a part of the pressure measurement subassembly of the sensor and is subsequently welded to the reference subassembly containing the ring and center electrodes. Welding the diaphragm in place, however, can cause the diaphragm to buckle thereby shifting the gap due to stresses in the weld. Thus, buckling introduces some non-parallelism since the diaphragm will no longer be completely planar. This is particularly critical for sensors that are used to measure relatively low differential pressures because of the very narrow gap required between the diaphragm and the ring and center electrodes.
Various techniques, such as electron beam welding and laser welding are known to provide welds having reduced stress, but such techniques are relatively expensive to implement. Using a relatively less expensive technique, such as Gas Tungsten Arc Welding (GTAW) or Plasma welding, results in greater stress being placed on the diaphragm. Similarly, reducing the thickness of the diaphragm to accommodate lower pressures is difficult because of the difficulty of securing the diaphragm relative to the housing without the diaphragm distorting. Accordingly, it is desirable to construct the sensor so that the diaphragm can be secured within the sensor housing in which the flatness of the diaphragm and the gap spacing between the diaphragm and each of the ring and center electrodes are satisfactorily maintained after the sensor is assembled.